The discovery that metallocenes of transition metals can be used as catalysts for the polymerization of olefins has led to significant amounts of research since it was found that different metallocenes could produce different types of polymers. One of the earliest references to the use of metallocenes in the polymerization of olefins is U.S. Pat. No. 2,827,446 which discloses a homogeneous, i.e. liquid, catalyst system of bis(cyclopentadienyl) titanium dichloride and an alkyl aluminum compound. The activity of such systems was not, however, as high as would be desired. It was latter discovered that more active catalyst systems would result if the metallocene was employed with an alkylaluminoxane cocatalyst, such as that disclosed in U.S. Pat. No. 3,242,099. One drawback of such metallocene-based catalyst system is that they generally must be used with large amounts of alkylaluminoxane cocatalysts, which are significantly more expensive than the alkyl aluminum cocatalysts. There is therefore a need for cocatalysts that would be effective in smaller amounts.
Various techniques are known for producing alkylaluminoxane, the simplest being to add water in a controlled fashion to an alkylaluminum compound such as disclosed in the aforementioned U.S. Pat. No. 3,242,099. Other techniques for producing such aluminoxanes involve reacting alkylaluminum compounds with solids containing water. See, for example, EPC 208,561; USSR Inventor Certificate 566,844: JP 60/289223; and U.S. Pat. Nos. 4,544,762; and 4,665,208.
For many commercial processes it is necessary to develop solid catalysts that are suitable for the particular type of commercial scale equipment employed. U.S. Pat. No. 4,431,788 teaches that a catalyst system can be prepared by reacting a solid having labile hydrogen atoms, such as starch, with an aluminum alkyl and then combining that product with a metallocene. Similarly German patent 3,240,382 teaches reacting alkyl aluminum compounds with solids containing water to produce solids coated with alkylaluminoxanes. Examples of other supported cocatalysts prepared by reacting organoaluminum compounds with solids containing water are disclosed in EPC 386,644; and U.S. Pat. Nos. 4,904,631; 4,912,075; 4,925,821; 5,006,500; 5,008,228; and 5,529,965.
The presence of soluble aluminoxane or polymerization catalyst, even on solid catalyst systems, has also been found to often be detrimental in commercial slurry type polymerization processes, the thought being that when the aluminoxane and/or polymerization catalyst is present in a dissolved form it contributes to the formation of fouling in the polymerization reactors. Accordingly, merely depositing aluminoxane on a solid support has not been found to be a particularly beneficial technique for preparing a catalyst system for slurry type polymerization processes, as some remaining soluble species usually result in reactor fouling.
A copending commonly owned U.S. patent application discloses a new type of catalyst system which uses a solid cocatalyst prepared by reacting a suitable support base with an organoaluminum compound and then reacting that product with an activity promoting amount of water. The solid catalyst is prepared by combining the solid cocatalyst with an olefin polymerization catalyst such as a metallocene and then conducting a prepolymerization to produce a solid catalyst system. The present invention is concerned with an improvement of that invention.